The separation of suspensions to concentrate or deplete suspended solids is a process that has many applications including, but not limited to, the treatment of water or other streams in industrial plants, municipal wastewater treatment plants, sand and gravel operations, and food processing. Separation can be accomplished by physical, chemical or electrochemical phenomena, and in some cases by a synergetic combination, such as chemical treatment prior to separation in wastewater treatment plants. As used herein, the suspension to be treated comprises solid matter suspended in a fluid, where the fluid may be a liquid or a gas. The separation of a suspension into its components can produce two outlet streams: one concentrated in solids, or dewatered if the fluid is water, and the other depleted of solids, or clarified. The processes of concentrating solids and clarifying liquids are closely related, with apparatus for clarifying or concentrating solutions sometimes differing according to which fluids are deemed valuable, and thus saved for use elsewhere.
A common physical basis for separation is the forcing of the suspension through a filtration membrane material, or filter, that does not allow suspended solids to pass. Common filtration techniques include forcing the suspension through a filter bag, either by flowing or squeezing the suspension through the bag, or placing the suspension on a filter medium and allowing the liquid to pass through, either under the force of gravity or by an applied force. Many practical problems arise in implementing these techniques in commercial or industrial processes. For example, since the material retained by the filter becomes more concentrated in solids than the unfiltered material, it usually become less fluid and thus more difficult to transport through the apparatus. Transport problems are usually handled through configuration that have a minimal amount of relative motion between the suspension and the filter medium. In addition, the concentrating of solids near the filter clogs the filter medium and thus hinders its filtering ability. Clogging is usually alleviated by flexing the filter medium. These problems limit the usefulness of the prior art by limiting the ability of the filter medium to act or by requiring frequent maintenance, or by incorporating additional complexity onto the system.
Prior Patents and Publications
The following patents and publications may be related to the invention or provide background information. Listing and discussion of these patents here should not be taken to indicate that any formal search has been completed or that any of these patents constitute prior art.
U.S. Pat. No. 5,614,092 to Di Leo is titled Filter for the Separation of Solids and Liquids from Muds and Specifically those from Industrial Processing. Di Leo discloses a filter bag for dewatering and clarifying muds. A filter bag is contained within a housing having a check valve for filling the bag with mud and an inflatable mechanism for squeezing a bag once it is filled. After the mud is introduced into the bag, the bag is sealed and then squeezed to force water through the filter bag, which is subsequently collected. The concentrate is either flushed out of the bag, or the bag is replaced. The invention of Di Leo characterizes some of the devices required to use filter bag technology to dewater or clarify suspensions. In many cases the force of gravity is insufficient to filter the suspension, especially when the fraction of solids is high, and an external force must be applied. The resulting condensate is typically difficult to transport and may require bag cleaning or disposal.
U.S. Pat. No. 5,725,783 to Hoden is titled Concentrating Method and Means Therefor. Hoden discloses a separation device that incorporates a permeable web conveyor belt. The belt is formed into a tube over part of the length of the travel by passing through a pair of folding devices that convert the belt from flat to tubular, and from tubular to flat. The suspension is introduced into the flat-to-tubular transition, and the concentrate is removed at the location of the tubular-to-flat transition. Separation of the liquid out of the belt occurs under the force of gravity or by squeezing the tube against a roller. While the invention of Hoden circumvents some of the problems encountered with the transport of concentrate by not introducing relative motion between the suspension and the filter medium, there are several problems with commercial implementation of this type of device. For example, the invention of Hoden has too many moving parts and uses belts that are both expensive and need to be replaced due to wear. In addition, there is the possibility of concentrate being squeezed out of the sides of the device, making recovery of concentrate difficult.
U.S. Pat. No. 4,348,290 to Schipper is titled Method and Device for Removing a Fluid from a Mixture of a Fluid and a Solid Substance. Schipper discloses a conveyor belt apparatus for separating a fluid from mixture of a fluid and a solid. The suspension is poured onto the conveyor belt which is permeable to the solids. The belt is forced between a roller and belt, and the concentrate is then allowed to fall off the end of the conveyor to be collected. The clarified suspension through the belt as it is forced against the roller. In Schipper, separation is achieved as the mixture is squeezed, and there is no relative motion between the mixture and the permeable belt. Although the transport of concentrate is solved by conveying the suspension, this is achieved at the expense of having multiple rollers with tensioning devices. In addition, the suspension can not be uniformly squeezed since it would be difficult to deal with mixtures near the edges of the belt. As with the invention of Hoden, Schipper also has too many moving parts and uses belts that are both expensive and need to be replaced due to wear. In addition, there is also the possibility of having to recover concentrate that is squeezed out of the sides of the device.
Many of the problems encountered in the prior art hinder the ability of dewatering or clarifying apparatus to operate effectively, increasing maintenance of filter media and other mechanical parts. What is needed is a method and apparatus that will continuously dewater and clarify a suspension with a minimum of mechanical components while continuously cleaning the filter medium.